A Very Large Scale Integration (VLSI) chip is generally composed of a silicon die having an integrated circuit printed thereon, a package for housing the silicon die which can be made of ceramic, organic or other types of chip carrier packages, and various types of electronic connection to the silicon die that extends to the exterior of the package for connection in an electronic system. Transistors and other circuit components reside on the silicon die and require power and signal connection extending to the exterior of the package. These connections would typically be coupled to a printed circuit board for further integration with other electronic components which are also located on the printed circuit board.
Referring to FIG. 1, one configuration of a conventional package 100 is shown. VLSI 102 is shown mounted on a ceramic base 104 and is connected via bond wires 106 to connection pins 108 through pin circuitry 112 of chip package 100. Ceramic base 104 is coupled to chip package 100 to dissipate heat from VLSI 102 out to the ambient environment outside chip package 100. Connection pins 108 are configured to mount on a circuit board (not shown) for further connection to other components.
As processing methods improve and designs become more advanced, every generation of VLSI chips continues to grow in complexity, performance and power consumption. As a result, the power and current demands for every generation of VLSI chips also increase. One of the biggest challenges for future generations of VLSI chips is managing the chip's power consumption.
AS shown in FIG. 2, the power consumption of a typical microprocessor is between 1 and 10 watts. However, as more complicated microprocessors are developed, more transistors are used, the size of the silicon die grows and the signal frequency greatly increases. Thus, as new generations of microprocessors are developed, however, the power demands are expected to increase into the hundreds or even thousands of watts as the complexity of the chip increases and as better chip performance is demanded.
One modern solution to manage increased power demand is voltage scaling. Voltage scaling is the process of reducing the voltage level of signals transmitted in VLSI chips so that less power is demanded. Power has a quadratic relationship to voltage where power is proportional to the square of the voltage. Hence, if the supply voltage is reduced by half, the power is reduced by one-fourth, giving a dramatic decrease in the power demand.
For example, in the 1980's, the typical power supply voltage was 5 volts. Later in the 1990's, the average supply voltage was reduced to 3.3 volts. More modern designs have reduced the supply voltage to as low as 2.5 volts and even 1.8 volts. Voltage scaling, however, has its limits and the continuing increase in power demands are still inevitable.
Unlike power, current is linearly proportional to voltage, according to Ohm's Law (V=IR). Therefore, if the supply voltage is reduced by half, the current is also reduced by half. Hence, voltage scaling only reduces supply currents by the same rate as reduction in voltage. Referring to FIG. 3, a logarithmic graph selected of microprocessor current demands over time is shown. For example, the Intel Corporation 80386 microprocessor had a current demand of less than 0.2 amperes. In 1989, the Intel 80486 microprocessor had twice the demand of the 80386 microprocessor of 0.4 amperes. Still further, the Pentium.TM. processor, available in 1993, had a much higher power demand of around 3 amperes. At this rate, according to the projected graph of FIG. 3, as time goes on, the current demand for microprocessors will greatly increase into the hundreds and even thousands of amperes. The net effect is that the power and current demands are, and will continue to be, major concerns in chip design.
With ever increasing power demands in next generation microprocessors and other VLSI's, heat dissipation is becoming another design concern. This concern is especially important with the emphasis of new designs moving towards multi-processor configurations.
FIG. 4 illustrates a conventional solution to power dissipation in a set of VLSI chip packages 404. Each overall package 404 is similar to that of FIG. 1 with the addition of cooling fins 402 mounted on base 412. The base 412 is typically made up of a material which allows for rapid heat dissipation such as ceramic or an organic or conductive material to help dissipate the heat through cooling fins 402. Set of VLSI chip packages 404 generates heat which is transferred to the base 412 for ultimate dissipation through the cooling fins 402. Each of set of VLSI chip packages 404 is typically mounted to a mother board 406 via set of pins 408.
Cooling fins 402 are usually exposed to the ambient temperature within a system such a computer chassis that may further provide fans and other cooling means to help dissipate the heat generated by the components in the system. For example, a modern more aggressive cooling technique is illustrated in FIG. 4 where a set of cooling fans 400 are located closely to a set of cooling fins 402 which are mounted on a respective ones of set of VLSI. As mentioned above, set of VLSI chip packages 404 are mounted to the mother board 406 via set of pins 408 for interconnection to the rest of the system (not shown). Set of cooling fins 402 is capable of dissipating a large amount of heat from set of VLSI chip packages 404.
As the rate of cooling is directly proportional to the difference in temperature, since the inside of a typical computer case is warm as a result of the heat generated by the collective components in the computer case, it will be useful to somehow expose set of VLSI chip packages to lower ambient temperature for more efficient heat dissipation.
One technique that has been used in the entertainment electronics industry for dissipating the heat generated from high power transistors has been to mount these transistors on the backside or other surface of a chassis. The surface on which the transistors are mounted is exposed to the ambient temperature outside of the chassis and, therefore, is able to dissipate heat from the heat generating transistors to the lower temperature of the outside environment.
However, to apply the technique of moving devices which are the heat generating culprits, such as microprocessors and other VLSI chips, away from their proximity to the other components on the main circuit board and place them on the surface of a chassis in order for cooling purposes could greatly degrade their performance in the system. As explained below, this is due to the use of a shared bus architecture in current computer systems.
Conventional computer systems connect components through a bus network that provides communication among subsystems such as a set of processors, memory sub-systems, input/output sub-systems and other sub-systems. Conventional methods call for mounting the microprocessor(s) on a motherboard, which is in the close proximity of the memory and peripheral components. Hence the microprocessor bus is short, thus fast. FIG. 5 is a general block diagram of such a prior art configuration where a set of microprocessors 500 are connected to a memory sub-system 502 and an input/output sub-system 504 via a set of connection stubs 506 on a system bus 508. System bus 508 is also known as a "stubbed bus".
In this configuration, set of connection bus stubs 506 allow each agent on system bus 508 (e.g., components such as set of micro-processors 500, memory sub-system 502, and input/output sub-system 504), to drive system bus 508 with a signal which is broadcasted to all of the agents sharing system bus 508. Every signal is broadcasted to all agents, but only the agent waiting for the appropriate signal responds to the signal.
Performance of the above configuration is greatly limited since system bus 508 is at risk of being overloaded when the agents connected to it become demanding, greatly degrading the performance. In addition, while operating frequencies of microprocessors have increased, the operating speeds of shared system buses have not increased to a significant amount. This is due to the difficulties in ensuring that the timing, impedence and other characteristics of the shared system bus remain within certain parameters.
In stubbed bus systems, short trace lengths between components must be maintained to ensure reliability and signal integrity. Thus, all sub-systems are generally placed in close proximity to achieve short trace lengths between the components of the stubbed bus. Thus, for a stubbed system bus, to try and separate components to any length is a difficult and expensive proposition.
The above restrictions of current shared system buses does not allow a designer to move heat generating components such as processors and other VLSI chips from a circuit board located inside a case to a surface on the case itself. In addition, when subsystems are placed on different surfaces, the subsystems will be operating with different ambient temperatures, as the subsystems which are located towards the center of the case will be at a different, most likely higher, temperature.
Thus, without being able to bypass the problems which would be presented by using a shared system bus if certain bus components were to be moved from the main circuit board to be mounted on a surface of a case, no benefit of the dissipation of heat through the use of the larger surface area of the case would be possible.